This invention relates to measuring means, and particularly to head position gauges in the field of magnetic recording and reproducing.
In so-called helical-scan magnetic tape transducing the heads must protrude just precisely the right distance from the surface of the rotating tape guide drum, in order that the tape stretch in the distorted "tent" in the head vicinity be uniform from head to head and from machine to machine; this is because variations in the degree of stretch cause variations in the wave-length of the recorded signal. But wear and accidental damage in the field can cause variations in the head protrusion, and a good gauge structure that can be used in the field and by relatively untrained operators has long been needed.
Previously known field-type gauges have usually been of the type mounting on some portion of the drum assembly, and must be carefully and precisely brought into engagement with each head in turn, to avoid damage to the delicate and brittle ferrite material of the head; and in some such gauges the head must be protected by means of a thin film or tape of plastic material, which of course introduces some indeterminate error into the measuring process. Such gauge structures usually also embody commercially available gauges of the type having an indicator dial and needle, and a pivoting spring-loaded ball-tipped lever arm for engaging one portion of the object to be measured. Since the ball tip cannot be effectively used in direct engagement with the head, which usually has a sloping surface, an intermediate bearing member must be interposed between the head and ball tip; and the coupling of the ball tip and the bearing member is often of a type that results in damage to the commercial gauge mechanism if the entire structure is dropped or roughly handled. Also, previous bearing members were usually made of steel, into which the hard ferrite material of the head is capable of gouging, at least to the degree that excessive pressure is placed on the more brittle ferrite, causing it to chip or fracture. In exacerbation of this effect, the spring-loading of the commercial gauge mechanism is usually quite strong, to overcome the internal friction and inertia of the gearing or other means that is needed to translate a motion of say one-thousandth of an inch at the head to a more visible movement of the dial-needle tip of about half-an-inch. Thus the bearing force of the ball-tip and bearing member against the head is too great, and fracture or chipping erosion of the head results, particularly at the head gap.
Accordingly, it is an object of the present invention to provide a gauge structure that is easily and accurately usable in the field without damage to a magnetic transducing head, or to the gauge structure itself.